1. National Institutes of Health, Office of Dietary Supplements, Bethesda, MD, USA
2. National Institute of Standards and Technology, Chemical Sciences Division, Gaithersburg, MD, USA
3. Imperial College Healthcare NHS Trust, London, UK
4. University of Liège, Centre Hospitalier Universitaire de Liège, Department of Clinical Chemistry, Liège, Belgium
5. The University of Western Australia, Centre for Microscopy, Characterisation and Analysis, Perth, WA, 6009 Australia
6. University College Cork, School of Food and Nutritional Sciences, Cork Centre for Vitamin D and Nutrition Research, Cork, Ireland
7. Loyola University Chicago, Department of Public Health Sciences, Chicago, IL, USA
8. University of Washington, Department of Laboratory Medicine, Seattle, WA, USA
9. Sonic Healthcare, Douglass Hanly Moir Pathology, Sydney, NSW, Australia
10. National Institute of Standards and Technology, Biomolecular Measurement Division, Gaithersburg, MD, USA
11. Health Canada, Nutrition Research Division, Ottawa, ON, Canada
12. National Institutes of Health, Office of Dietary Supplements, Bethesda, MD, USA ,National Institute of Standards and Technology, Chemical Sciences Division, Gaithersburg, MD, USA